I suggest you had better go to the ophthalmology of the hospital, and ask a professional doctor to undertake measuring pupillary distance. Because the doctors are professional, the data obtained are authoritative and credible.
You may do it yourself, if it is inconvenient for you to go to the hospital or optician. You can measure the pupillary distance at home, you should take the ruler that has marks, ask your family member to over your eyes, at this moment you can not move your eyes, read out directly from left eye pupil to the distance between right eye pupil, however be sure to repeat at least 3 times to make it accurate.
If you have worn glasses before, you can make a measurement based on the original glasses data. Generally speaking, an optical center can be found on the prepared lenses by means of the instrument. Between the optical centers of the two lenses is the data of PD.